Platelet P2T- purinergic receptors are unique among P2-purinergic receptors since ADP is the preferred agonist and ATP an antagonist. Preliminary studies indicate that at least two cell lines that express P2T-purinergic receptors have been identified. The specific aims are: 1) To identify a model cell line for the study of the P2T-purinergic receptors. 2) To determine the signalling mechanism(s) and the G-protein(s) and phospholipase C involved in the activation of the P2T-purinergic receptors. 3) To clone the cDNA encoding the receptor from a human megakaryocytic cell line. 4) To stably express the cloned P2T- purinergic receptor(s) to establish its signalling mechanisms and to provide a model for eventual development of more potent and specific agonists and antagonists.